Modify the `default` suite to use the `saltmajorversion` InSpec input
rather than the `version` variable in `pkgs_spec.rb`.

Also:

* add keyring file for the salt repo, due to `apt-key` deprecation
* remove old repository file

Use the following command:

```
$ GREP_SED="^\(\s\+master_type: \)true" \
  && grep -rl "${GREP_SED}" test/integration/ \
  | xargs sed -i -e "/${GREP_SED}/s//\1str/"
```

Use the following command:

```
$ GREP_SED="\(repo\.salt\)stack\.com" \
  && grep -rl --exclude-dir=.git "${GREP_SED}" \
  | grep -v .un~$ \
  | xargs sed -i -e "/${GREP_SED}/s//\1project.io/g"
```

```
$ GREP_SED="\(pkgrepo: deb http\):" \
  && grep -rl "${GREP_SED}" test/integration/v* \
     | xargs sed -i -e "/${GREP_SED}/s//\1s:/"
```

* CVE-2021-25283 enables Jinja2 safe mode, which breaks use of
  `'dict' in x.__class__.__name__` workaround
* Workaround no longer needed as CentOS 6 is EOL